1. Field of the Invention
This invention relates to a process for making a film comprising a combination of a film making process, an inflation process for molding an annular original film, a tenter process for stretching the annular original film in the width direction, and particularly to a process for making a film in which a porous film suitable to be used as a raw material of disposable sanitary goods, such as a paper diaper or the like, can be made at a low cost and with ease.
2. Description of the Prior Art
As a representative process of a horizontal stretching or biaxial stretching of a film or a sheet (hereinafter represented by the "film"), there have heretofore been a tenter process or a flat process (hereinafter represented by the "tenter process") and a tubular process (hereinafter including the "mandrel process"). There are also processes for molding an original film suitable for the respective stretching processes.
As a process for forming an original film when the film is stretched in accordance with the tenter process, a die process for forming a flat film, a casting process, or a flat process (hereinafter represented by the "T-die process") is adopted. Similarly, as a process for forming an original film when the film is stretched in accordance with the tubular process, an inflation process for forming an annular film is adopted.
The tenter process is a process in which the entire film is uniformly stretched by clamping both ends of a wide film. As the quality of a film stretched in accordance with the tenter process depends on the accuracy of the thickness before the film is stretched, the T-die process having a very high accuracy in thickness of the film is adopted for forming an original film. However, the equipment cost of the T-die process molding apparatus is high compared with the inflation molding apparatus.
On the other hand, the tubular process is a process in which an annular original film is stretched in accordance with the inflation process utilizing the sealing pressure of air which is to be charged into a tube. The tubular process has a strong correlation with respect to the vertical and horizontal stretch ratio. Therefore, there exists such a problem that the vertical and horizontal stretch ratio of a film stretched in accordance with the tubular process are difficult to obtain be set to desired optional results.
Also, as a process for making a porous film or a porous sheet (hereinafter represented by the "porous film") among films, there is a process for making a porous film having a membrane structure (communicating hole structure) by forming an original film by dispersing an inorganic or organic filler in a resin raw material which will act as a base, and thereafter this original film is stretched to generate cracks around the filler as a core.
However, when a porous film is to be made according to the above-mentioned process, selection of fillers to be dispersed in the resin and contents thereof become important factors in making such a porous film. In addition, a process for molding an original film and a process for stretching the same also become very important factors. Accordingly, the selection of these factors in the process for making a porous film becomes important in determining the physical properties of the porous film. Particularly, when a porous film having a large horizontal strength is to be obtained, a horizontal stretching or a biaxial stretching is required. In this case, a process for molding the original film becomes particularly important.
When an original film is to be molded in accordance with the inflation process, the conventional process for molding an original film is limited to the tubular process or the mandrel process in which a film is stretched in its annular shape. Similarly, when a flat-shaped original film is to be molded in accordance with the T-die process, it is limited to the tenter process in which a film is stretched in its flat-shape. Particularly, with an inexpensive porous film employed in sanitary goods, such as a paper diaper or the like, the film is stretched in the horizontal direction in order to increase strength in the horizontal direction. In this case, there is a correlation between the molding process and the stretching process. Regarding the annular original film containing a filler and a flat original film containing a filler, a porous film has been made by means of a combination of the above-mentioned molding process and stretching process. This can be seen with respect to a stretching process which is described in Japanese Patent Publication No. Sho 60-26009.
In this way, it has been commonly known that the mandrel process is applied to the annular original film which is made in accordance with the inflation process and the T-die process is applied to the flat original film which is obtained in accordance with the T-die process.
Accordingly, the physical properties of films obtained by the respective processes have their own characteristics. On the other hand, in the above-mentioned combined process, a target property of a film is difficult to set due to its nature and there can be obtained a film of limited physical properties depending on the respective processes for molding the original film. Particularly, in a case of an original film obtained in accordance with the inflation process, a correlation between the horizontal stretch ratio and the vertical stretching multiple factors is significant and optional results are difficult to obtain, as mentioned above.